test(browser): empty media folders by a rename, delete them in the background (#384)

With the database commits gone, throttled resets still stalled in rmtree:
unlinking the last fixture's files waits for their pending writeback. Each
reseed now moves the old output and input folders into <root>/reset-trash
and makes empty ones; a background thread deletes the trash, and the
server's cleanup stops it and removes the rest before the temp root goes.

tests/test_browser_folder_trash.py

@@ -0,0 +1,52 @@
+"""Browser-test media folders are emptied by a rename, deleted later (#384)."""
+
+import tempfile
+import threading
+import unittest
+from pathlib import Path
+
+from tests.browser.folder_trash import TRASH_NAME, close_trash, replace_folder
+
+
+class TestFolderTrash(unittest.TestCase):
+    def setUp(self):
+        self.tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(self.tmp.cleanup)
+        self.root = Path(self.tmp.name)
+        self.addCleanup(close_trash, self.root)
+
+    def trash_threads(self):
+        return [t for t in threading.enumerate() if t.name == "pm-reset-trash"]
+
+    def test_replaces_the_folder_with_an_empty_one(self):
+        folder = self.root / "output"
+        (folder / "nested").mkdir(parents=True)
+        (folder / "nested" / "image.png").write_bytes(b"png")
+        replace_folder(folder)
+        self.assertTrue(folder.is_dir())
+        self.assertEqual(list(folder.iterdir()), [])
+        close_trash(self.root)
+        self.assertFalse((self.root / TRASH_NAME).exists())
+        self.assertEqual(self.trash_threads(), [])
+
+    def test_a_missing_folder_is_created_without_a_trash(self):
+        replace_folder(self.root / "input")
+        self.assertTrue((self.root / "input").is_dir())
+        self.assertFalse((self.root / TRASH_NAME).exists())
+        self.assertEqual(self.trash_threads(), [])
+
+    def test_repeated_replacements_share_one_thread_and_all_are_removed(self):
+        folder = self.root / "output"
+        for number in range(5):
+            folder.mkdir(exist_ok=True)
+            (folder / f"{number}.png").write_bytes(b"png")
+            replace_folder(folder)
+        self.assertEqual(len(self.trash_threads()), 1)
+        close_trash(self.root)
+        self.assertEqual(list(folder.iterdir()), [])
+        self.assertFalse((self.root / TRASH_NAME).exists())
+        close_trash(self.root)  # idempotent
+
+
+if __name__ == "__main__":
+    unittest.main()